Fix poseidon hashing when absorbing empty slice

Author: sebastiencs

ledger/src/poseidon/mod.rs

@@ -1872,6 +1872,23 @@ impl<F: Field, SC: SpongeConstants> Sponge<F, F> for ArithmeticSponge<F, SC> {
 
     #[inline(never)]
     fn absorb(&mut self, x: &[F]) {
+        if x.is_empty() {
+            // Same as the loop below but doesn't add `x`
+            match self.sponge_state {
+                SpongeState::Absorbed(n) => {
+                    if n == self.rate {
+                        self.poseidon_block_cipher();
+                        self.sponge_state = SpongeState::Absorbed(1);
+                    } else {
+                        self.sponge_state = SpongeState::Absorbed(n + 1);
+                    }
+                }
+                SpongeState::Squeezed(_n) => {
+                    self.sponge_state = SpongeState::Absorbed(1);
+                }
+            }
+            return;
+        }
         for x in x.iter() {
             match self.sponge_state {
                 SpongeState::Absorbed(n) => {

ledger/src/scan_state/transaction_logic.rs

@@ -945,6 +945,9 @@ pub mod zkapp_command {
     pub struct Event(pub Vec<Fp>);
 
     impl Event {
+        pub fn empty() -> Self {
+            Self(Vec::new())
+        }
         pub fn hash(&self) -> Fp {
             hash_with_kimchi("MinaZkappEvent", &self.0[..])
         }
@@ -8395,6 +8398,16 @@ mod tests {
             .into_compressed()
     }
 
+    #[test]
+    fn test_hash_empty_event() {
+        // Same value than OCaml
+        const EXPECTED: &str =
+            "6963060754718463299978089777716994949151371320681588566338620419071140958308";
+
+        let event = zkapp_command::Event::empty();
+        assert_eq!(event.hash(), Fp::from_str(EXPECTED).unwrap());
+    }
+
     /// Test using same values as here:
     /// https://github.com/MinaProtocol/mina/blob/3a78f0e0c1343d14e2729c8b00205baa2ec70c93/src/lib/mina_base/receipt.ml#L136
     #[test]